Change RFC6979 implementation to be a generic PRNG #269

1. 
   sipa commented at 10:14 pm on July 8, 2015: contributor

   Instead of making it take key, message, and extra data separate, just pass in a byte array of seed data, turning it into just a non-EC specific PRNG.

   The usage inside the blinding code and the tests is also adapted to make use of this in a more natural way.

2. sipa cross-referenced this on Jul 10, 2015 from issue Implement Schnorr signatures by sipa
3. 
   sipa commented at 11:19 pm on July 16, 2015: contributor

   @gmaxwell Want to review?
4. 
   dcousens commented at 0:59 am on July 17, 2015: contributor

   @sipa maybe its taken care by secp256k1_rfc6979_hmac_sha256_initialize, but RFC6979 specifies:

   If that value of k is within the [1,q-1] range, and is suitable for DSA or ECDSA (i.e., it results in an r value that is not 0; see Section 3.4), then the generation of k is finished. The obtained value of k is used in DSA or ECDSA.

   Is this handled externally [to that function] as part of the libraries signing process? In which case, I assume you feed T back in somehow? If you don’t feed it back, and just restart, then technically this isn’t RFC6979 compliant.

5. 
   sipa commented at 9:18 pm on July 18, 2015: contributor

   @dcousens Right, secp256k1_rfc6979_hmac_sha256 only implements the PRNG side of the construction. The specific use as defined by RFC6979 is done inside the signing code.
6. 
   apoelstra commented at 4:21 pm on July 23, 2015: contributor

   With benchmarks on

   0src/bench_internal.c: In function ‘bench_rfc6979_hmac_sha256’:
   1src/bench_internal.c:268:9: error: too many arguments to function ‘secp256k1_rfc6979_hmac_sha256_initialize’
   2         secp256k1_rfc6979_hmac_sha256_initialize(&rng, data->data, 32, data->data, 32, NULL, 0);
   3         ^
   4In file included from src/bench_internal.c:11:0:
   5src/hash_impl.h:205:13: note: declared here
   6 static void secp256k1_rfc6979_hmac_sha256_initialize(secp256k1_rfc6979_hmac_sha256_t *rng, const unsigned char *key, size_t keylen) {
   7             ^
   8make: *** [src/bench_internal-bench_internal.o] Error 1
   

7. sipa force-pushed on Jul 24, 2015
8. 
   sipa commented at 1:50 pm on July 24, 2015: contributor

   Rebased and fixed bug found by @apoelstra.
9. 
   gmaxwell commented at 5:16 pm on July 24, 2015: contributor

   ACK. (but see nit)
10. 
    apoelstra commented at 6:44 pm on July 24, 2015: contributor

    tested ACK
11. Change rfc6979 implementation to be a generic PRNG 3e6f1e20dc
12. sipa force-pushed on Jul 24, 2015
13. 
    sipa commented at 8:13 pm on July 24, 2015: contributor

    Addressed nit.
14. sipa merged this on Jul 24, 2015
15. sipa closed this on Jul 24, 2015

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16. sipa referenced this in commit 4c63780710 on Jul 24, 2015

Contributors
sipa dcousens apoelstra gmaxwell